1. Field of the Invention
The present invention relates to an ion type smoke sensor in which a change of ion current at a smoke sensing part is detected by a field effect transistor (FET).
2. Prior Art
One example of this ion type smoke sensor is disclosed in U.S. Pat. No. 3,733,596. In order to reduce power consumption and to avoid or extremely lessen an erroneous alarm issuance of the sensor due to the invasion of insects into the sensor, an oscillating circuit such as a free running multivibrator is provided to intermittently pulse-drive an insulated gate field effect transistor (MOSFET) for sensing smoke. FIG. 1 shows a circuit construction of a conventional ion type smoke sensor, in which an oscillating output from an oscillator circuit 1 is applied to the base of a transistor 2 so as to render the transistor 2 conductive intermittently. The emitter of the transistor 2 is connected to a positive side power source line L1 and the collector is connected to a negative side power source line L2 by way of a series circuit including resistors R1 and R2. The resistors R1 and R2 constitute a voltage divider having a voltage dividing point producing a voltage pulse with a predetermined level and with a frequency corresponding to the oscillating output from the oscillator circuit 1. The voltage pulse is applied to the voltage dividing point of a voltage divider including resistors R3 and R4 connected in series between the power source lines L1 and L2, through a coupling capacitor C1. Accordingly, the voltage dividing point provides an output signal which is formed by superposing the above-mentioned voltage pulse onto a reference voltage defined by the resistors R3 and R4. The output signal as a result of this superposition is applied to the source electrode S of a field effect transistor (referred to as FET hereinafter) 3. The drain electrode D of the FET 3 is connected to the negative side power source line L2 via a resistor R5. The gate electrode G of the FET 3 is connected to an intermediate electrode 4c of a smoke sensing element 4. The external electrode 4a of the smoke sensing element 4 is connected to the positive side power source line L1 and the internal electrode 4b of the smoke sensing element 4 to the negative side power source line L2. The source potential of the FET 3 intermittently changes in accordance with the oscillating frequency of the oscillator 1 in response to the superposed output derived from the voltage divider having the resistors R3 and R4. At the same time, the gate G of the FET 3 receives the gate signal depending on the ion current of the smoke sensing element 4. As a result, a fire detecting signal is intermittently derived from the drain electrode D of the FET 3 via the output terminal 5.
In the circuit arrangement shown in FIG. 1, the FET 3 has a large capacitance and an impedance of the ion chamber forming the smoke sensing element 4 is very high. As a result, the output impedance of the smoke sensing element 4 with respect to the gate electrode G is high. Accordingly, if the FET 3 is driven directly in pulse mode, the operation of the FET 3 is not stable and consequently the fire sensing output is not stabilized.